PLL with Lock-in Frequency Controller

ABSTRACT

A PLL has a frequency comparator that is active during lock-in. It outputs a signal related to the difference between the oscillator frequency and a target frequency. It captures an initial phase and observes change in phase relative to the initial phase. Two ways of capturing the initial phase are provided. The frequency comparator can provide input signals for the loop filter and make the PLL act as a frequency-locked loop during lock-in. Alternatively, it can provide input signals for a search controller that may perform a binary or other search. The frequency comparator may wait one or more cycles of the reference clock signal to reduce noise, or it may set a threshold to eliminate some noise. It may signal that the oscillator frequency equals the target frequency when the threshold has not been exceeded after a timeout. The search controller may directly or indirectly control the PLL&#39;s oscillator.

CROSS REFERENCES TO OTHER APPLICATIONS

This application claims priority from U.S. Provisional PatentApplication Ser. No. 62,671,822, entitled Digital Circuits and Methodsfor Phase-Locked Loops, filed on May 15, 2018, which is herebyincorporated by reference as if set forth in full in this applicationfor all purposes.

This application is related to the following application, U.S. PatentApplication Serial No. <serial no>, entitled Phase Accumulator withImproved Accuracy, filed concurrently herewith, which is herebyincorporated by reference, as if set forth in full in thisspecification.

This application is related to the following application, U.S. PatentApplication Serial No. <serial no>, entitled Power-Saving PhaseAccumulator, filed concurrently herewith, which is hereby incorporatedby reference, as if set forth in full in this specification.

This application is related to the following application, U.S. PatentApplication Serial No. <serial no>, entitled PLL with Beat-FrequencyOperation, filed concurrently herewith, which is hereby incorporated byreference, as if set forth in full in this specification.

This application is related to the following application, U.S. PatentApplication Serial No. <serial no>, entitled PLL with Phase RangeExtension, filed concurrently herewith, which is hereby incorporated byreference, as if set forth in full in this specification.

BACKGROUND

The present invention relates generally to electronic circuits used togenerate clock signals and more specifically to phase-locked loops(PLLs) manufactured in deep-submicron integrated circuit (IC)technologies.

Phase-Locked Loops are circuits that produce an output clock signalwhose phase can be locked to the phase of an input reference clocksignal. Phase, in the context of a PLL, means a signal's frequency valueintegrated over a time period. Therefore, the phase of the signal equalsthe number of clock cycles during the time period. The ratio of thefrequency of the output clock signal and the frequency of the referenceclock signal can be a positive integer number, in which case the PLL iscalled an integer-N PLL or an “integer PLL”; or it can be a positiverational number, in which case the PLL is called a fractional-N PLL or a“fractional PLL”. Rational numbers are numbers that can be expressed asa ratio of two integers. In the context of this document, a fractional-Nnumber is a positive rational number consisting of an integer part(obtained by rounding down to the nearest integer number) and afractional part.

A PLL includes a controlled oscillator that produces the output clocksignal. An analog PLL usually has an analog-controlled oscillator suchas a voltage-controlled oscillator (VCO), and a digital PLL may have ananalog-controlled oscillator or a digitally-controlled oscillator (DCO),which may include a digital-to-analog converter followed by ananalog-controlled oscillator. A PLL locks the phase (and, as a result,frequency) of the output clock signal to the phase of the referenceclock signal by measuring the accumulated number of output clock cyclesand adjusting the controlled oscillator frequency when the measurednumber deviates from a required (or predicted) number, based on afrequency control word (FCW). The ratio of output clock cycles toreference clock cycles, measured over some duration, is called the PLL'smultiplication factor. When a conventional PLL is in lock, itsmultiplication factor matches its FCW and its output frequency matches atarget frequency defined as the reference clock frequency times the FCW.

Frequency-Locked Loops (FLLs) are circuits that produce an output clocksignal whose frequency is locked to the frequency of an input referenceclock signal. Compared to a PLL, an FLL lacks the integration oraccumulation over time. A PLL's integration may occur anywhere in itsloop, for example in feedback circuits, or in feedforward circuits suchas a loop filter. Whereas a PLL in lock will lock both frequency andphase ratios in output and reference signals, an FLL may lock just thefrequency ratio but not necessarily the phase ratio.

In an integer-N PLL, the spacing of possible output clock frequencies(also called the frequency resolution) typically equals the frequency ofthe reference clock signal, since the output clock frequency equals apositive integer number times the reference clock frequency. Increasingor decreasing the positive integer number by one will result in theoutput clock frequency increasing or decreasing by the referencefrequency. A better output frequency resolution can be achieved by usinga lower reference clock frequency. However, in practical PLLs this willoften increase the jitter.

A fractional-N PLL can have a much better output clock frequencyresolution without the need for a low reference clock frequency, as theratio between the output clock and reference clock frequencies can be apositive rational number. An example of a fractional-N PLL is describedin U.S. Pat. No. 8,994,523, entitled Phase-Locked Loop Apparatus andMethod by Jenkins. The circuits described there provide very highaccuracy and low jitter. However, new applications require ever higherspeed, higher accuracy, a larger range, lower jitter, and lower power.To meet these requirements simultaneously, embodiments of the inventionincorporate novel architectures and methods.

SUMMARY

At startup, or any other time prior to acquiring lock, a PLL's signalsin its loop may have random or default startup values, or values basedon prior conditions that are no longer valid or relevant. Therefore, itmay be expected that its controlled oscillator will oscillate at anoscillator frequency far different from a target frequency based on thereference clock signal and a configured FCW. In a conventional PLL,lock-in may be slow. PLLs according to embodiments of the inventionprovide a frequency comparator that outputs a signal related to adifference between the target frequency and the oscillator frequency.Embodiments use this signal to preset an oscillator control code oroscillator control signal, and/or other PLL parameters, to quickly setthe PLL in a state close to lock.

An embodiment of the invention provides a PLL with a phase predictor, aphase subtractor, and a frequency comparator. The frequency comparatoris active during the lock-in period. It outputs a signal related to adifference between the target frequency and the oscillator frequency bycapturing an initial phase and observing a change in phase relative tothe initial phase. A first way of capturing the initial phase is loadinga value of a sampled phase that represents the initial phase in thephase predictor, such that subsequent predictions provided by the phasepredictor include the change in phase relative to the initial phase. Asecond way of capturing the initial phase is loading a value of thephase difference into a register calculating the change in phaserelative to the initial phase by subtracting the value in the registerfrom subsequent phase differences. The frequency comparator may observemerely a sign of the change in phase relative to the initial phase todetermine if the oscillator frequency is higher or lower than the targetfrequency. It may wait for one or more cycles of the reference clocksignal before observing the change in phase relative to the initialphase. Alternative embodiments output the signal if a magnitude of thechange in phase relative to the initial phase exceeds a threshold. Whena timeout occurs, an embodiment may output a signal that the oscillatorfrequency equals the target frequency.

An embodiment may include a search controller that searches for anoscillator control signal that results in an oscillator frequency withina search margin of the target frequency. The search controller may usesuccessive output signals of the frequency comparator. It may performany type of search, including a binary search, a linear search, a jumpsearch, an interpolation search, an exponential search, a Fibonaccisearch, and a Newton-Raphson search. The search controller may include afilter. In embodiments, it may set parameters of the loop filter, andother parameters in the PLL's loop.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be described with reference to the drawings, inwhich:

FIG. 1 illustrates a digital PLL according to an embodiment of theinvention;

FIG. 2 illustrates some errors occurring in a phase accumulator;

FIG. 3 illustrates a phase accumulator with reduced counter errorsaccording to an embodiment of the invention;

FIG. 4 illustrates an example sequence with one bit change per countaccording to an embodiment of the invention;

FIG. 5 illustrates a phase accumulator according to an embodiment of theinvention;

FIGS. 6A-C illustrate additional error sources in the phase accumulatorof FIG. 5;

FIG. 7 illustrates a phase accumulator combining the features of FIGS. 3and 5 according to an embodiment of the invention;

FIG. 8 illustrates a method to measure a PLL's output clock phase withimproved accuracy;

FIG. 9 illustrates detail of a power-saving phase accumulator accordingto an embodiment of the invention;

FIG. 10 illustrates an analysis of beat frequency locking according toan embodiment of the invention;

FIG. 11 shows an analysis of a one-octave DCO frequency range and itsfrequency lock ranges based on K=R_(N)/2 according to an embodiment ofthe invention;

FIG. 12 shows a table with normalized frequency lock ranges versusselection of K according to an embodiment of the invention;

FIG. 13 illustrates a first beat-frequency PLL according to anembodiment of the invention;

FIG. 14 illustrates a method for operating a beat-frequency PLLaccording to an embodiment of the invention;

FIG. 15 illustrates a method for operating a beat-frequency PLL near orat a required frequency according to an embodiment of the invention;

FIG. 16 illustrates a second beat-frequency PLL according to anembodiment of the invention;

FIG. 17 illustrates a PLL with a frequency comparator according to anembodiment of the invention;

FIG. 18 illustrates an alternative PLL with a frequency comparatoraccording to an embodiment of the invention;

FIG. 19 illustrates a first method to predict a frequency differenceprior to PLL lock according to an embodiment of the invention;

FIG. 20 illustrates a second method to predict a frequency differenceprior to PLL lock according to an embodiment of the invention;

FIG. 21 illustrates pseudo code for a PLL with range extension accordingto an embodiment of the invention;

FIG. 22 illustrates alternative pseudo code for a PLL with rangeextension according to an embodiment of the invention;

FIG. 23 illustrates an example PLL with range extension for an arbitraryrange value K according to an embodiment of the invention;

FIG. 24 illustrates how a PLL can alternatively provide range extensionaccording to an embodiment of the invention;

FIG. 25 illustrates pseudo code for the example in FIG. 24 for a PLLwith range extension according to an embodiment of the invention;

FIG. 26 illustrates an example PLL with range extension for the methodin FIGS. 24 and 25 according to an embodiment of the invention;

FIG. 27 illustrates generalized circuitry for phase difference rangeextension according to an embodiment of the invention; and

FIG. 28 illustrates a generalized method to achieve phase differencerange extension according to an embodiment of the invention.

DETAILED DESCRIPTION

Examples in this Detailed Description may refer to the use of adigitally controlled oscillator (DCO), which may be controlled by anoscillator control code (OCC). All such examples equally apply for theuse of an otherwise controlled oscillator, such as a voltage-controlledoscillator (VCO) or current-controlled oscillator (CCO), which may becontrolled by an oscillator control signal (OCS). Embodiments employinga VCO, CCO, OCS are within the scope and ambit of the invention, unlessexpressly excluded.

Phase-Locked Loops (PLLs) are circuits that produce an output clocksignal whose phase can be locked to the phase of a reference clock inputsignal. Phase, in the context of a PLL, means a signal's frequency valueintegrated over time, i.e. the signal's number of accumulated clockpulses. The ratio of the frequency of the output clock signal and thefrequency of the reference clock signal can be a positive integernumber, in which case the PLL is called an integer-N PLL or “integerPLL”, or it can be a positive rational number, in which case the PLL iscalled a fractional-N PLL or “fractional PLL”. Rational numbers arenumbers that can be expressed as a ratio of two integers. In the contextof this document, a fractional-N number is a positive rational numberconsisting of an integer part (obtained by rounding down to the nearestinteger number) and a fractional part.

A digital PLL may measure the output clock phase, and compare themeasured phase with a required or predicted phase. The required ratiomay be configured or expressed by a frequency control word (FCW). Theratio of oscillator output clock cycles to input reference clock cyclesis called the PLL's multiplication factor. When a conventional PLL is inlock, its multiplication factor matches its FCW. Embodiments of theinvention perform measurement of the output phase in a phaseaccumulator. A phase predictor calculates a required phase, based on thereference clock and an FCW. The embodiments perform phase comparisonusing a digital subtractor (or adder). They filter the comparison resultin a loop filter, and use the loop filter output to control theoscillator.

FIG. 1 illustrates a PLL 100 according to an embodiment of theinvention. PLL 100 receives reference clock signal 101 and producesoutput clock signal 103. The frequency ratio of output clock signal 103and reference clock signal 101 is determined by a FCW 102 that mayinclude an integer and a fractional part. A controlled oscillator 110generates output clock signal 103. Controlled oscillator 110 may be, orinclude, a digitally-controlled oscillator (DCO) or an otherwisecontrolled oscillator, such as a VCO or CCO. A phase accumulator 120measures the output clock signal 103 phase by counting and/or measuringfull and, if needed for accuracy, partial cycles of output clock signal103. Therefore, the phase accumulator 120 output signal isrepresentative of the output clock signal 103 phase. Upon receiving areference clock signal 101 pulse, register 130 samples the output clocksignal 103 phase, stores it, and makes it available at its output as themeasured phase. This output value of register 130 is representative ofthe output clock signal 103 phase at the time of the most recentreference clock signal 101 active edge. Register 130 may include a setof parallel latches, for example D-latches with enable and data inputs,each latch for one bit of the output clock signal 103 phase, each latchtriggered by reference clock signal 101 to measure the output clocksignal 103 phase, and each latch providing a bit of measured phase atthe register 130 output. In some embodiments, register 130 is locatedinside and/or part of phase accumulator 120, and in other embodiments itmay be external to phase accumulator 120 but receive its input signalfrom phase accumulator 120.

While examples of further details of phase accumulator 120 will beprovided later in this document, several embodiments are possible.Perhaps the simplest embodiment is just a modulo-K counter. A modulo-Kcounter, for the purposes of this patent document, is a counter thatcounts a maximum of K pulses at an input, providing a unique countedvalue at an output, and that restarts at a start value upon countingevery K^(th) pulse. A modulo-K counter may count up or count down. Thestart value may be zero, or any other value. The counted value may beexpressed as a binary number, or as any other representation of anumber. Examples in this patent document are based on up-counting from avalue 0 to a value K−1, after which the counter restarts at 0. However,mutatis mutandis, all examples are equally valid for counters countingdown and/or starting at different values. The value of K may be fixed,or it may be programmable, or it may be made to vary over time.

A phase predictor 140 takes FCW 102, and upon receiving a referenceclock signal 101 pulse, phase predictor 140 calculates a predictedphase. In its most simple form, the predicted phase equals a number ofreference clock signal 101 cycles received by phase predictor 140,multiplied by FCW 102. In more sophisticated embodiments, phasepredictor 140 applies noise shaping to move lower-frequency truncationnoise to higher frequencies. The operation of phase predictor 140 hasbeen described in detail in other documents, for example in U.S. Pat.No. 8,994,523, entitled Phase-Locked Loop Apparatus and Method byJenkins. A phase subtractor 150 calculates a phase difference betweenthe predicted phase at the phase predictor 140 output and the measuredphase by subtracting the measured phase from the predicted phase. A loopfilter 160 filters the phase difference to produce an oscillator controlsignal forwarded to controlled oscillator 110 to control the frequencyof output clock signal 103. Loop filter 160 may be, or include, ananalog filter, for example when the controlled oscillator 110 is a VCO,or it may be, or include, a digital filter, for example when controlledoscillator 110 is a DCO.

A PLL has a multiplication factor whose maximum value is determined bythe highest possible controlled oscillator frequency divided by thelowest allowed reference frequency. The ratio includes integer numberR_(N), where R_(N) stands for the range of integer numbers N.Traditionally, a fractional-N FCW includes integer number N, which mayhave any integer value up to R_(N). It further includes a fraction of 1that may be expressed as an integer value M below a maximum R_(M), whereR_(M) stands for the range of the fraction of 1. When in lock, the PLL100 multiplication factor is f_(DCO)/f_(REF)=(N+M/R_(M)). A shorthandversion of this is f_(DCO)/f_(REF)=FCW. Some embodiments of theinvention have a different multiplication factor, as will be explainedlater with reference to FIG. 10.

The phase accumulator 120 and phase predictor 140 need to be able totrack and predict the phase of output clock signal 103 over asufficiently large range. An embodiment may provide equal ranges forphase accumulator 120 and phase predictor 140, or ranges that aredifferent from each other. For phase accumulator 120 this range equals avalue K: for example, upon reaching a counted value of K−1 output clockcycles, a counter inside phase accumulator 120 continues counting at 0.Traditionally, the range K needs to be large enough to count R_(N)cycles of output clock signal 103 during one cycle of reference clocksignal 101. To correct large phase errors, or to relock at the correctphase after lock has been temporarily lost, traditional PLLs use a largerange K, for instance K>2R_(N). This means that the range K of thecounter (a modulo-K counter) is at least two times as large as a maximumnumber of output clock signal 103 cycles R_(N) during a reference clocksignal 101 cycle. Thus, the maximum value of a phase difference is notlimited by the period of reference clock signal 101, but by the largerof K and a range of phase predictor 140. The range of phase predictor140 is usually chosen to match the range K of phase accumulator 120.Some embodiments of the invention are capable of using K values that aresmaller than R_(N).

Phase Accumulator with Improved Accuracy

FIG. 2 illustrates some errors occurring in a phase accumulator. Thephase accumulator includes counter 220 and register 230. Jitterrequirements in some PLLs can be as low as 100 femto-seconds (10⁻¹³S).For a semiconductor foundry process in which a simple logic gate canhave a rise or fall time in the order of 10 pico-seconds (10⁻¹¹S), evena small manufacturing variation can have a large impact. Manufacturingvaries because its opto-lithographic production process uses light toetch structures whose size is much smaller than the light's wavelength.The error sources are both in counter 220 and register 230. For example,counter 220 receives a DCO clock signal which may have a frequency farabove clock speeds that are used in an integrated circuit (IC) forclocking the rest of its logic. Therefore, the logic gates providing thecounter 220 output bits need to respond very fast, and in fullalignment. However, due to mismatching and variations in setup and holdtime, some gates will change later than others, and errors are likelyespecially when many gates change their bit value at the same time. Forexample, in a conventional twelve-bit counter, all bits may need tochange at the same time when the count goes from 2,047 to 2,048 (01111111 1111 to 1000 0000 0000). However, if not all bits switch at thesame time, any individual bit can be in error and hence a value samplednear the time of transition may result in a number that is neither2,047, nor 2,048. As all bits are changing, the result may be any valuebetween 0 and 4095. The result may be not at all close to the correctvalue. Similarly, register 230 must respond very fast—but used at theirtop speed, some input gates may not capture fast-changing input bits intime. This problem occurs especially when a PLL's reference clock andDCO clock active edges occur very close to each other.

FIG. 3 illustrates a phase accumulator 300 with reduced counter errorsaccording to an embodiment of the invention. It includes counter 320,register 330 (that may include latches), and binary encoder 335. Itsolves or almost fully eliminates the problem described above withreference to FIG. 2 that occurs when counter 320 bits do not all respondequally fast, or bits of register 330 do not all respond equally fast.Counter 320 features an output sequence that changes only one bit percount. An example of such a sequence is the Gray code, so counter 320may include a Gray counter. Since instead of potentially all bits onlyone bit changes, the chance of an error is much reduced. Moreover, if aone-bit change is not captured by register 330, then the error is nevermore than one count, and it is always in the same direction. Generallythe design of the remainder of the PLL may not be based on such a codebut, for example, on a simple binary code, and therefore phaseaccumulator 300 may comprise binary encoder 335, which converts the codeused in counter 320 to the code used by relevant other parts of the PLL.For example, if counter 320 includes a Gray counter, then binary encoder335 may include a Gray-to-binary converter. Some embodiments of a PLLthat includes phase accumulator 300 may further provide for detectionand correction of the error.

FIG. 4 illustrates an example sequence 400 with one bit change per countaccording to an embodiment of the invention. The example sequence 400 isnot a Gray code. It is simple to implement with a bit valued “1” thatripples from the right (least significant bit side) to the left (mostsignificant bit side) until it bumps against a most significant bit, oragainst another bit valued “1”. When all entries are filled with “1”s,the most significant bit toggles, and the sequence unwinds in reverse.The reverse second half of the sequence guarantees that the one-bitchange per count is maintained when it is implemented in a modulo-Kcounter. If example sequence 400 is implemented in a modulo-K counter,its K-value is 48, and in the transition from count 47 to count 0, onlythe most significant bit toggles.

Although the example sequence 400 has a K value that is lower than thelimit of a binary or a Gray counter, and may therefore use more powerthan necessary, the redundancy can be used for error detection andcorrection. While example sequence 400 is shown with a width of 6 bits,this class of sequences can be implemented with any bit width. Andalthough it is provided as an example where a bit valued “1” ripples, itcan also be implemented as a sequence where a bit valued “0” ripples. Orit can be implemented as a sequence where bits ripple from the left tothe right. The example shown has bits alternatingly rippling bythemselves or in a group of 2. In other implementations, other groupsizes are possible.

FIG. 5 illustrates a phase accumulator 500 according to an embodiment ofthe invention. Phase accumulator 500 includes delay line 510, counter520, which may include a modulo-K counter, first latches 540, secondlatches 530, delay line decoder 550, and optional calibrator 570. Acontrolled-oscillator clock signal 503 triggers both delay line 510 andcounter 520, whereas reference clock signal 501 triggers both secondlatches 530 and first latches 540. Second latches 530 and first latches540 fulfill the function of register 130 in FIG. 1.

When phase accumulator 500 receives a controlled-oscillator clock signal503 pulse, counter 520 increments, and the pulse starts propagatingthrough delay line 510. The delay line 510 has a number of stages, whoseaverage delay determines its resolution, and a time range that is longerthan the longest duration of a controlled-oscillator clock signal 503cycle. It includes multiple output taps, matching the number of stages,and the output taps are coupled to second latches 530. When a pulsepropagates through delay line 510, the pulse also propagates along itsmultiple output taps. Counter 520 counts cycles of controlled-oscillatorclock signal 503, extending the time range of phase accumulator 500 toits maximum value (K−1 in the case of a modulo-K counter) times theshortest duration of a controlled-oscillator clock signal 503 cycle.Counter 520 has multiple output bits (e.g., ²log K), that are coupled tofirst latches 540. When phase accumulator 500 receives a reference clocksignal 501 pulse, second latches 530 sample the multiple delay line 510output taps, and first latches 540 sample the counter 520 output bits.Thus, the output state of second latches 530 freezes a state ofpropagation of the most recent controlled-oscillator clock signal 503pulse at the time of receiving the reference clock signal 501 pulse, andfirst latches 540 freezes a number of controlled-oscillator clock signal503 pulses received by counter 520. The first latches 540 output signalincludes a code produced by counter 520 that signifies an integer partof a controlled-oscillator clock signal 503 measured phase, the integeroutput phase 505. The second latches 530 output signal includes a seriesof successive equal bit values (1s or 0s) of which the front signifiesthe fractional progress of the most recent controlled-oscillator clocksignal 503 pulse. The delay line decoder 550 converts this to a binarynumber representing the fractional output phase 504. Delay line decoder550 may be implemented with, for example, combinatorial logic or amemory that includes a lookup-table.

Embodiments forward fractional output phase 504 to optional calibrator570, which may perform a calibration at, e.g., startup aftercontrolled-oscillator clock signal 503 has initially stabilized, to seta required target number of delay line steps per controlled-oscillatorclock signal 503 pulse. Optional calibrator 570 may further perform abackground calibration during phase accumulator 500 operation to keepthe required number of delay line steps per controlled-oscillator clocksignal 503 pulse stable regardless of changes in operating conditionssuch as temperature and supply voltage. Optional calibrator 570 may doso by repeatedly measuring the actual number of delay line steps percontrolled-oscillator clock signal 503 pulse and averaging the resultsto obtain an accurate number. Optional calibrator 570 may form anegative feedback loop with delay line 510, second latches 530, anddelay line decoder 550 that keeps the average actual number of delayline steps per controlled-oscillator clock signal 503 pulse equal to therequired actual number of delay line steps per controlled-oscillatorclock signal 503 pulse.

FIGS. 6A-C illustrate additional error sources in the phase accumulatorof FIG. 5. FIG. 6A shows an example of correct alignment between delayline 510 and counter 520. Each controlled-oscillator clock signal 503cycle (538, 539, 540, . . . ) ripples through exactly 13 stages (0-12)of delay line 510. The response of delay ine 510 is fully aligned withthe counter 520 response. However, since the resolution of the delayline is determined by its design, the IC manufacturing technology,process variations, the temperature, and the applied voltage, whereasthe controlled-oscillator clock signal 503 cycle time may be dependenton unrelated factors, it is generally not easy to guarantee an exactpredetermined number of delay line 510 steps per controlled-oscillatorclock signal 503 cycle (unless an embodiment includes optionalcalibrator 570). FIG. 6B shows that in practical situations the numberof delay line 510 steps per controlled-oscillator clock signal 503 cyclemay not be an integer. In this example, step 12 is a partial step. FIG.6C shows that delay line 510 and counter 520 may not be aligned either.Generally, delay line 510 will have an offset in time, meaning thatdelay line 510 will respond to a controlled-oscillator clock signal 503cycle earlier or later than counter 520. As explained in U.S. Pat. No.9,484,889, “Delay Fabric Apparatus and Delay Line”, by Julian Jenkins,this offset can be intentional to improve linearity.

FIG. 7 illustrates a phase accumulator 700 combining the features ofFIGS. 3 and 5 according to an embodiment of the invention. Phaseaccumulator 700 includes counter 720 which features an output sequencechanges only one bit per count. It includes a delay line, first latches,a delay line decoder, first latches 740, binary encoder 760, and acalibrator. Phase accumulator 700 receives reference clock signal 701and controlled-oscillator clock signal 703. It outputs integer part 705and fractional part 704 of the measured phase. The sequence of counter720, first latches 740, and binary encoder 760 ensures that the integerpart 705 of the measure phase is never off by more than one count. Thecalibrator ensures that the number of fractional steps per controlledoscillator cycle is correct.

FIG. 8 illustrates a method 800 to measure a PLL's output clock phasewith improved accuracy. The method includes the following steps:

Step a—in a phase accumulator, receiving the PLL's output clock andcounting output clock pulses in a counter whose output value changesonly one bit per counted output clock pulse. The counter may, forexample, be or include a Gray counter.

Step b—in a register or in latches, upon receiving a reference clockpulse, sampling and storing the counter output value.

Step c—(optional) converting the output value stored in the register orlatches to a binary number.

Power-Saving Phase Accumulator

FIG. 9 illustrates detail of a power-saving phase accumulator 900according to an embodiment of the invention. Since counter 520 in FIG. 5needs to be very fast, it may consume much power. FIG. 9 shows that itcan be split in a fast counter 920 and a low-power counter 925.Similarly, the first latches 540 of FIG. 5 is split in first latches 940and third latches 945. The fast counter 920 is triggered by clock signal903, which may come from a controlled oscillator, whereas the low-powercounter 925 is triggered by the fast counter 920 carry signal.Therefore, the fast counter 920 output value represents one or moreleast significant bits of the integer clock phase 905, whereas thelow-power counter 925 output value represents the remaining one or moremost significant bits of the integer clock phase 905. Both first latches940 and third latches 945 are triggered by reference clock signal 901.From the discussion around FIG. 2 it is clear that this may lead toerrors, especially when a reference clock signal 901 occurs right aftera carry signal from fast counter 920 to low-power counter 925, andbefore low-power counter 925 has been able to change its output signal.However, this error can be corrected in optional binary encoder 960 iffast counter 920 and/or low-power counter 925 change no more than onebit per count. If only fast counter 920 but not low-power counter 925changes no more than one bit per count, optional binary encoder 960 maybe concatenated with first latches 940, but not with third latches 945,and vice versa. Yet another embodiment could use two different suchsequences for fast counter 920 and low-power counter 925, in which casefirst latches 940 would have a first binary encoder, and third latches945 would have a second binary encoder.

Splitting the counter and latches into fast and low-power circuits hasthe advantage that the most significant bits in a phase count, in thesplit counter, consume much less power than least significant bits inthe phase count. For example, if the fast counter 920 is one bit wide,bits in low-power counter 925 can theoretically save up to 50% energyfor each time they switch. However, they would switch relatively often,and more energy may be saved by making fast counter 920 two bits wide(increasing its power consumption by 50%), which would make low-powercounter 925 bits theoretically save up to 75% energy for each time theyswitch, and they switch 75% less often. Of course, the energy savingsmay not match the theoretical maximum. A designer can find an optimumsplit between the number of bits for fast counter 920 and low-powercounter 925 by taking all relevant factors into account, and/orsimulating candidate configurations.

Power-saving phase accumulator 900 may further include a delay line,second latches, and a delay line decoder, which all operate similar todelay line 510, second latches 530, and delay line decoder 550 toproduce a fractional clock phase 904 signal.

PLL with Beat-Frequency Operation

In prior-art PLLs great care was taken to ensure that they had a singlefrequency lock range (FLR), covering the full DCO frequency range.Whatever a DCO frequency was prior to locking, given a reference clocksignal in an allowed range, and a frequency control word (FCW), thePLL's loop would force the DCO frequency to become the FCW times thereference clock signal frequency. Therefore, in prior-art PLLs, thephase accumulator and a phase predictor range were chosen to be largeenough to capture at least twice a PLL's multiplication factor range,i.e., K>2R_(N), where R_(N)=f_(DCP_MAX)/f_(REF_MIN). For example, to beable to multiply a minimum reference frequency f_(REF_MIN) of 0.5 MHz toa maximum DCO frequency f_(DCO_MAX) of 1,024 GHz (i.e., R_(N)=2048), atleast a 12-bit counter (K=2¹²=4096) was used. During one cycle of a 0.5MHz reference clock, the counter would count 2,048 cycles of a 1.024 GHzDCO clock, and the counter would show an output value at half itsmaximum value. If the DCO wasn't in lock yet, the counter could show avalue as low as 0, corresponding to a DCO frequency that is lower thanthe 0.5 MHz reference clock frequency, and it could show a value as highas 4,095, corresponding to an (out-of-range) DCO frequency of 4.095 GHz.

Had K been chosen to be smaller than R_(N), the PLL could also lock toDCO frequencies higher than FCW *f_(REF). For example, if the above PLLwith R_(N)=2048 had an 8-bit counter (K=256), its highest FCW would be255 (i.e., K−1). With a 0.5 MHz reference clock and an FCW=255 it couldlock to 127.5, 255.5, 383.5, 511.5, 639.5, 767.5, 895.5, and 1023.5 MHz.It would have eight separate FLRs, and the DCO could come to lock ineach one, dependent on its prior state. Every 128 MHz there would be aso-called “beat frequency” to which the DCO could lock. Beat frequenciesare spaced K times the reference frequency apart (see analysis below,with reference to FIG. 10). Locking to a beat frequency was consideredundesirable. But embodiments of the present invention use this behaviorto their advantage for saving power. A modulo-K counter and register(second latches) in a phase accumulator are very high-speed circuits.High speed comes at the cost of high power usage, and reducing the bitwidths of the modulo-K counter, second latches, phase predictor andphase subtractor saves power.

FIG. 10 illustrates an analysis of beat frequency locking according toan embodiment of the invention. FIG. 10 shows a simplified diagram of aPLL 1000 that is in lock. The analysis is equally valid for an integer-Nand a fractional-N PLL. The diagram leaves out anything that is notrelevant for the analysis, including the loop filter. PLL 1000 includesDCO 1010, modulo-K counter 1020, phase predictor 1030, and phasesubtractor 1040. DCO 1010 generates a DCO output signal 1002 withfrequency foco, which it forwards to modulo-K counter 1020. Modulo-Kcounter 1020 counts cycles of DCO output signal 1002, whose values aresampled at each cycle of reference clock signal 1001, which has afrequency f_(REF). The modulo-K counter 1020 output signal represents ameasured phase 1004. Phase predictor 1030 has a predicted phase 1003output signal, whose value is updated at each cycle of reference clocksignal 1001. Since PLL 1000 is in lock, the predicted phase 1003 andmeasured phase 1004 are equal, and phase difference 1005 equals zero.The predicted phase 1003 has a value (t*f_(REF)*FCW) mod K, in which tstands for the time (this analysis ignores any noise shaping inpredicted phase 1003). The product t*f_(REF) represents the accumulatednumber of reference clock signal 1001 cycles over the time t. This ismultiplied by the FCW to get predicted phase 1003, and the result ispresented modulo-K to match the output format of modulo-K counter 1020.The measured phase 1004 has a value of (t*f_(DCO)) mod K. Here,t*f_(DCO) represents the accumulated number of DCO output signal 1002cycles over the time t.

Since predicted phase 1003 and measured phase 1004 are equal,(t*f_(REF)*FCW) mod K=(t*f_(DCO)) mod K. This should be valid for anytime t, including for the first time t₁ that modulo-K counter 1020 andphase predictor 1030 are sampled by reference clock signal 1001, i.e.,t₁=1/f_(REF). The left-hand side mod K can be ignored, because it onlymaps higher frequencies onto f_(REF). The right-hand side mod K can berewritten as −(li−1)*K with li∈{1, 2, . . . }, so that the formulabecomes: FCW=(f_(DCO)/f_(REF))−(li−1)*K. Rearranged, the multiplicationfactor N becomes:

N=f _(DCO) /f _(REF)=FCW+(li+1)*K with li∈{1, 2, . . . }.

As expected, the fundamental lock frequency for f_(DCO) is atFCW*f_(REF). Beat frequencies occur above that at equally spacedK*f_(REF) intervals.

A perspective less dependent on formulas is as follows. If the maximumcounter value (e.g., K−1) is reached at the maximum DCO frequency(f_(DCO_MAX)) during one cycle of the minimum reference frequency(K=R_(N)), and the DCO has an ideal frequency range from 0 tof_(DCO_MAX), then there is only one DCO frequency for which measuredphase 1004 could match predicted phase 1003, and it equals FCW*f_(REF).However, if R_(N) is larger than K, say n times as large, then modulo-Kcounter 1020 will loop n times during the one cycle of the minimumreference frequency to count the R_(N) cycles of DCO 1010 running at itsmaximum frequency. However, this implies that there are n differentfrequencies where the measured phase 1004 will match the predicted phase1003, and PLL 1000 could lock to any of those frequencies. The DCO 1010ideal frequency range from 0 to f_(DCO_MAX) is divided into n separateFLRs, each of which has exactly one match for a selected FCW. The sizeof each FLR is f_(DCO_MAX)/n, and it also equals K*f_(REF). The value off_(DCO_MAX) may not be tightly controllable, since it may be dependenton manufacturing variations, the temperature, and the DCO 1010 powersupply (voltage or current), and therefore n and R_(N) are not knownaccurately. However, embodiments are in full control of K, and thechoice of the index li for selecting a correct FLR.

A FLR with index li starts at: f_(FLR_MIN)(li)=(li−1)*K*f_(REF)

It ends at: f_(FLR_MAX)(li)=li*K*f_(REF)

Target DCO frequency within the range:f_(DCO_TARGET)(li)=(li−1)*K*f_(REF)+FCW*f_(REF)

PLLs do not usually have an ideal frequency range from 0 to f_(DCO_MAX).One reason is the difficulty of designing a DCO with such a wide range.Instead, a DCO may have a range of just over an octave or part of anoctave (one octave means that the minimum DCO frequency is half themaximum DCO frequency). If the range is one octave, then an applicationcan obtain any lower frequency by using an output divider. If DCO 1010has a frequency range of ½ f_(DCO_MAX) to f_(DCO_MAX), for K=R_(N)/2(i.e., n=2), there will be only 2 separate FLRs. The first one, for thefundamental lock frequency (li=1), is unavailable because it is below ½f_(DCO_MAX). The second one, with the first beat frequency (li=2), isavailable because it fully covers the DCO frequency range.

FIG. 11 shows an analysis of a one-octave DCO frequency range and itsFLRs based on K=R_(N)/2 according to an embodiment of the invention. Thevertical arrow represents a linear frequency spectrum 1100, starting at0 Hz (at the bottom) and going to the maximum DCO output frequencyf_(DCOMAX) at the top. The minimum DCO output frequency f_(DCOMIN) is anoctave lower, so at ½ f_(DCOMAX). Since n=2, there are two FLRs, eachwith a size of ½ f_(DCOMAX). The first lock range is located between 0and ½ f_(DCOMAX), and the second lock range is located from ½ f_(DCOMAX)to f_(DCOMAX). The first lock range, associated with the fundamentalfrequency (li=1), is fully out of range of the DCO. However, the secondlock range, associated with the first beat frequency (li=2), fullycovers the DCO frequency range. Therefore, embodiments of the inventionusing a one-octave DCO can always safely use the first beat frequency.No matter what frequency the DCO may have in its one-octave range priorto achieving lock, when n=2 it will always lock to the first beatfrequency. Similarly, a half-octave DCO with n=4 will always lock to thethird beat frequency (li=4), and so on. Embodiments in which the DCOfrequency range covers multiple FLRs must take care that a DCO frequencyat the start of a lock-in period is already within the target frequencyrange, otherwise the embodiment could lock to a wrong beat frequency.They can do so in a few ways, for example: (1) achieving initial lockwith a different mechanism, such as a frequency-lock loop; (2) achievinginitial lock with a sufficiently high K, and decreasing K once initiallock has been achieved; (3) narrowing the DCO range prior to achievinglock; or (4) presetting the DCO at a frequency guaranteed to be in thedesired FLR, for example at the DCO minimum, middle, or maximumfrequency, prior to acquiring initial lock. A frequency-lock loop, forexample, may comprise counters to measure the DCO frequency and thereference clock frequency, and a feedback loop to set the DCO frequencyclose to li*K+FCW times the reference clock frequency, with li>1.Presetting the DCO at a frequency guaranteed to be in the desired FLRmay require preprogramming registers in the loop filter to provide acontrolled DCO behavior at the start of a lock-in period.

FIG. 12 shows a table with normalized FLRs versus selection of Kaccording to an embodiment of the invention. It shows how, for a PLLwith R_(N)=2,048 and a normalized maximum DCO frequency of 1,000, thenormalized FLR (“range”) reduces with decreasing numbers of counterbits. At K=R_(N)=2,048, only the fundamental frequency can be used.There is only one lock range (“#bands”) and its normalized size is1,000. Reducing the number of counter bits, for example, to 5 (K reducesto 32), there are up to 64 FLRs for the fundamental and up to 63 beatfrequencies, and each FLR has a normalized width of 15.6.

FIG. 13 shows a first beat-frequency PLL 1300 according to an embodimentof the invention. Some PLLs are designed for applications that requireonly a narrow, controlled oscillator frequency range and that also havea minimum reference frequency—sometimes dictated by an industrystandard, or a jitter or phase noise requirement. For example, a PLLthat locks to a crystal—including a jitter attenuator that locks to acrystal—may need to show a low-phase noise or low-jitter output signal.To prevent that a controlled oscillator that includes an LC tank becomesdominant for the phase noise or jitter, the loop filter cut-offfrequency needs to be sufficiently high, and therefore the referencefrequency needs to be sufficiently high, for example above 100 kHz orabove 1 MHz. A beat-frequency PLL can save power, and may be as easy tooperate as a regular PLL.

First beat-frequency PLL 1300 includes a controlled oscillator 1310configured to generate an output clock signal 1302 with a maximum outputclock frequency. It further includes phase accumulator 1320 comprising amodulo-K counter with an input receiving output clock signal 1302. Thephase accumulator 1320 has a maximum K value. Phase accumulator 1320outputs a measured phase signal 1303 that is sampled by register 1330upon receiving an active edge of reference clock signal 1301 which has aminimum reference clock frequency. First beat-frequency PLL 1300 furtherincludes phase predictor 1340 which is configured to calculate arequired phase based on a FCW upon receiving an active edge of referenceclock signal 1301, and phase comparator 1350 which calculates a phasedifference by subtracting the sampled measured phase at the register1330 output from the required phase. To generate an oscillator controlsignal, loop filter 1360 filters the phase difference. Phase accumulator1320 and phase predictor 1340 have a range of output signals (themeasured phase signal 1303 and the required phase) from 0 to K1−1,wherein K1 is the modulo-K value with the maximum K value. Unlike inconventional designs, first beat-frequency PLL 1300 has a maximum Kvalue that is less than a multiplier range value defined as the ratio ofthe maximum output clock frequency over the minimum reference clockfrequency. For example, the maximum K value may be half the multiplierrange value. In some embodiments, controlled oscillator 1310 has adifference between the maximum output clock frequency and a minimumoutput clock frequency that is smaller than a frequency lock range (FLR)whose size equals K1 times the minimum reference clock frequency. Infurther embodiments, K1 can be modified and made smaller than themaximum K value.

First beat-frequency PLL 1300 may have a means for setting the outputclock frequency in a target FLR prior to the first beat-frequency PLLachieving final lock. Whereas some embodiments have an output clockfrequency range is fully included in a single frequency lock range (FLR)and are therefore guaranteed to lock to a desired frequency, in otherembodiments the output clock frequency range spans multiple FLRs. One ofthese, the target FLR, includes the desired frequency. Should controlledoscillator 1310 oscillate in an FLR other than the target FLR during thelock-in period, then the first beat-frequency PLL 1300 will lock to anundesired beat frequency. To guarantee that first beat-frequency PLL1300 locks to the desired frequency, it needs to set the output clockfrequency in the target FLR prior to lock-in. Several means forachieving this have been discussed with reference to FIG. 11. Theyinclude (1) using a frequency-lock loop, which can be much faster than aregular PLL loop; (2) using a wider FLR spanning the full output clockfrequency range during a provisional lock-in; (3) comparing the ratio ofoutput clock frequency and reference frequency and using a binary searchto determine an oscillator control code that yields the requiredmultiplication factor prior to lock-in; and (4) presetting thecontrolled oscillator at a value known (by design) to be in the targetFLR.

FIG. 14 illustrates a method 1400 for operating a beat-frequency PLLaccording to an embodiment of the invention. Method 1400 includes thefollowing steps.

Step 1410—Determining multiple FLRs covering a controlled oscillatorfrequency range. An embodiment can save power by lowering the ranges ofthe beat-frequency PLL phase accumulator, register, phase predictor, andphase subtractor. Lowering the ranges may result in increasing thenumber of FLRs. Although the beat-frequency PLL could achieve lock ineach of the multiple FLRs, it will only lock to a frequency in an FLRwithin which the controlled oscillator is already operating.

Step 1420—Based on a required frequency, determining a target FLR. Thetarget FLR is one of the multiple FLRs and the target FLR includes therequired frequency. Therefore, this method step comprises determiningwhich one of the multiple FLRs includes the required frequency.

Step 1430—Operating the controlled oscillator at a frequency in thetarget FLR. Method 1400 prepares the controlled oscillator to oscillatenear or at the required frequency before its final settings are ineffect. The controlled oscillator frequency needs to be in the same FLRas the required frequency. One method of operating the controlledoscillator near the required frequency (i.e., in the target FLR) is bypresetting the controlled oscillator, for example at its highestfrequency, at its lowest frequency, in the middle of its range, or atany other frequency known to be in the target FLR. This method isconvenient when the number of final FLRs is low and each final FLR has alarge bandwidth. Another method to operate the controlled oscillator inthe target FLR will be described with reference to FIG. 15.

Step 1440—Determining a final K-value related to the multiple FLRs and afinal FCW to generate the required frequency in the target FLR.

Step 1450—Setting the final K-value and the final FCW and operating thebeat-frequency PLL to generate the required frequency in the target FLR.

FIG. 15 illustrates a method 1500 for operating a beat-frequency PLL ata required frequency (i.e., in a target FLR) according to an embodimentof the invention. Method 1500 includes the following steps.

Step 1510—Determining a single FLR covering the full controlledoscillator frequency range. By having the full controlled oscillatorfrequency range covered by the single FLR, the embodiment guaranteesthat the controlled oscillator will be able to lock to only the requiredfrequency.

Step 1520—Determining an initial K-value related to the single FLR andan initial FCW to generate the required frequency in the single FLR.

Step 1530—Setting the initial K-value and the initial FCW and operatingthe beat-frequency PLL to generate the required frequency in the singleFLR.

An example of applying the above methods for setting a beat frequencyPLL is as follows. Suppose the beat frequency PLL receives a referencefrequency of 1 MHz and has a required output frequency of 539 MHz, sothe required multiplication factor N=539. Suppose further that the beatfrequency PLL has a DCO frequency range of 500 MHz to 1 GHz, and aminimum acceptable reference frequency of 0.5 MHz, so its R_(N)=2,000.Because it has a one-octave DCO, it supports K up to about ½ R_(N), i.e.the maximum value of K is 1024, and the widths of its phaseaccumulator's modulo-K counter, second latches, phase predictor, andphase subtractor are all up to 10 bits in support of a maximum K of1024. Suppose that power savings analysis has shown that the most powercan be saved with a K of 4, then the beat frequency PLL supports aminimum K of 4, i.e. the bit widths all reduce to 2 bits.

The beat frequency PLL is set to acquire initial lock first. Since thereference frequency is 1 MHz, an embodiment can reduce K to 512,maintaining n=2. The beat frequency PLL uses only 9 of the 10 bits inthe phase accumulator, phase predictor, and phase subtractor. To achievea multiplication factor N=539, it programs the FCW at 27. Since it usesthe first beat frequency (li=2), N=FCW+(li−1)*K=27+(2−1)*512=539. Usingthese settings, the beat frequency PLL achieves initial lock and the DCOsettles at a frequency of 539 MHz. Now that the DCO is at the rightfrequency, it is possible to use a much narrower FLR, and reduce Kfurther. Since most power is saved with K=4, the embodiment reduces Kfrom 512 to 4, reducing bit widths from 9 bits to 2 bits. It alsochanges the FCW to maintain N at 539. The new FCW is 3. The embodimentuses the 134^(th) beat frequency (li=135), so that N=3+134*4=539. TheFLR is reduced from the initial 500 MHz (calculated from (K=)512*(f_(REF)=) 1 MHz) to 4 MHz (calculated from (K=) 4*(f_(REF)=) 1MHz).

The K value may or may not be a factor of 2. For example, a counterpresenting the sequence in FIG. 4 has a K value of 48, which is not afactor of 2. However, a binary counter may have a width of b bits, and aK value of 2^(b) (two to the power of b). In this special case, while anew K is derived from the previous K by limiting the modulo-K counterbit width, a new FCW can be derived from the previous K by limiting itsbit width and effectively masking its most significant bits. Forexample, in binary code, 539 reads as 010 0001 1011. Reducing the bitwidth from 11 to 2 reduces the binary code to 11, i.e. 3 as a decimalnumber.

FIG. 16 illustrates a second beat-frequency PLL 1600 according to anembodiment of the invention. It comprises controlled oscillator 1610,phase accumulator 1620, register 1630, phase predictor 1640, phasecomparator 1650, loop filter 1660, and lock detector 1670 configured toprovide phase lock indicator signal 1608. In some embodiments, lockdetector 1670 has an input coupled with a phase comparator 1650 output,as drawn. In other embodiments, lock detector 1670 may receive an inputsignal from elsewhere, such as from phase accumulator 1620. Register1630 and phase predictor 1640 have inputs coupled with a reference clocksignal 1601 input, and controlled oscillator 1610 provides an outputclock signal 1603 to phase accumulator 1620. Controlled oscillator 1610has an output clock frequency within a frequency range. Phaseaccumulator 1620 includes a modulo-K counter, which is configured forreceiving output clock signal 1603 and counting output clock signal 1603cycles up to a maximum modulo-K counter K−1 value at which it restartscounting at 0. The modulo-K counter is configured to switch from a firstK value (K1) related to a first bit width to a second K value (K2)related to a second bit width at a start of a modulo-K cycle when thephase lock indicator signal 1608 indicates that the output clockfrequency is within a target FLR, and from the second K value to thefirst K value at the start of a modulo-K cycle when the phase lockindicator signal 1608 indicates that the output clock frequency is notwithin the target FLR. Modulo-K counter provides an output phase countvalue 1604 to register 1630, which samples it upon receiving a referenceclock signal 1601 pulse and stores a measured phase count value andprovides it at its output until it receives another reference clocksignal 1601 pulse and updates it. Register 1630 is configured to switchbetween the first bit width and the second bit width simultaneously withthe modulo-K counter switching between the first K value and the secondK value. Phase predictor 1640, upon receiving a reference clock signal1601 pulse, calculates a predicted phase count value, which includes anumber of reference clock signal 1601 cycles multiplied by a FCW. Phasepredictor 1640 is configured to switch between a first FCW, the first Kvalue, the first bit width and a second FCW, the second K value, and thesecond bit width simultaneously with the modulo-K counter switchingbetween the first K value and the second K value. Phase comparator 1650calculates a phase difference by subtracting measured phase count valuefrom predicted phase count value. Loop filter 1660 receives phasedifference and filters it to provide an oscillator control code tocontrolled oscillator 1610.

In some embodiments, the modulo-K counter K−1 value can be increased ordecreased. Further embodiments may also change a bit width of phasecomparator 1650. Even further embodiments may change the modulo-Kcounter K−1 value and the register 1630 bit width by splitting themodulo-K counter in two or more separate counters with different speedgrades and splitting register 1630 in two or more registers withdifferent speed grades (as discussed with reference to FIG. 9) andswitching off or on the slower blocks.

In some embodiments, lock detector 1670 receives the phase differenceand determines if successive values of phase difference are within anarrow band around zero to indicate the second beat-frequency PLL 1600is in phase lock, and upon determining that second beat-frequency PLL1600 is in phase lock, it asserts phase lock indicator signal 1608. Infurther embodiments, phase lock indicator signal 1608 includes a measureof phase lock tightness which the embodiments use to adjust the modulo-Kcounter K−1 value.

Other embodiments may use different methods of lock detection. Forexample, they may check on the presence of a reference clock signal 1601pulse during a short period that such a reference clock signal 1601pulse is expected. Or they may use a delta-frequency predictor signal toindicate lock or loss of lock.

PLL with Lock-In Frequency Controller

FIG. 17 illustrates a PLL 1700 with a frequency comparator 1770according to an embodiment of the invention. PLL 1700 further includescontrolled oscillator 1710, phase accumulator 1720, register 1730 (whichin some embodiments is included in phase accumulator 1720), phasepredictor 1740, phase subtractor 1750, loop filter 1760, searchcontroller 1780 and multiplexer 1790. Some embodiments may implementfrequency comparator 1770, loop filter 1760, search controller 1780 andmultiplexer 1790 as a single block, for example in the form of a digitalprocessor dedicated to frequency processing and control. PLL 1700 isconfigured to receive a reference clock signal 1701 and to deliver anoutput clock signal 1703.

At startup or any other time when PLL 1700 is not in lock, signals inthe loop may have random or default startup values, or values based onprior conditions that are no longer valid or relevant. Therefore, it maybe expected that controlled oscillator 1710 will oscillate at anoscillator frequency rather different from a target frequency based onthe reference clock signal 1701 and a configured FCW. During a lock-inperiod, embodiments search for a controlled oscillator 1710 oscillatorcontrol signal (ocs) value that makes controlled oscillator 1710oscillate at or close to the target frequency (generally, within asearch margin), for example by performing a binary or other search, orby operating as a frequency-locked loop. Frequency comparator 1770 isconfigured to output frequency difference signal 1708, which is orincludes a signal related to the difference between the target frequencyand the oscillator frequency. It captures an initial phase, and observesa change in phase relative to the initial phase.

As the example of a binary search, an embodiment may first programcontrolled oscillator 1710 to oscillate midway its frequency range, forexample by setting an oscillator control code (that may be included inan oscillator control signal) most significant bit (msb) to 1 and allless significant bits to 0. It then compares the resulting outputfrequency with the target frequency. If the target frequency is higherthan the measured frequency, the target frequency will be located in theupper half of the controlled oscillator 1710 frequency range, and if itis lower than the measured frequency, it will be located in the lowerhalf of the controlled oscillator 1710 frequency range. The embodimentmay thus deduce that the msb is a 1 or a 0, respectively. The embodimentthen sets the next-significant bit to 1, thereby making the controlledoscillator 1710 oscillate midway the higher or lower half of thefrequency range, respectively. Again, if the target frequency is higherthan the measured frequency, this next-significant bit must be 1, and ifit is lower than the measured frequency, this next-significant bit mustbe 0. The embodiment may repeat this process until all bits of theoscillator control code have been determined and the controlledoscillator 1710 frequency is within the search margin or at the targetfrequency. An embodiment may use a binary search as described here; alinear search; a jump search; an interpolation search; an exponentialsearch; a Fibonacci search; a Newton-Raphson; or any other search methodknown in the art. The embodiment may use merely the sign of thefrequency difference, and thus find one bit of the oscillator controlcode per search cycle, or it may use both sign and amplitude anddetermine more than one oscillator control code bit per cycle. Once thesearch process has been completed, the embodiment copies relevant valuesinto the various signals and provides controlled oscillator 1710 theoscillator control signal, which allows PLL 1700 to achieve phase lockmuch faster, since the oscillator frequency is already (close to)correct. The process is as follows.

At the start of a lock-in period, multiplexer 1790 allows searchcontroller 1780 to provide controlled oscillator 1710 an oscillatorcontrol signal. In response, controlled oscillator 1710 will oscillateat a corresponding frequency, and phase accumulator 1720 will count ormeasure output clock signal 1703 clock cycles. The result is sampled byregister 1730 when a reference clock signal 1701 pulse arrives, and madeavailable at its output as sampled phase 1705. Frequency comparator 1770issues search control signal 1709 to phase predictor 1740, which thenloads sampled phase 1705 in an internal register to serve as an initialphase value for the predicted phase 1706. Phase subtractor 1750subtracts the initial phase from predicted phase 1706 and produces phasedifference signal 1707. Successive values of phase difference signal1707 expose a change in phase relative to the initial phase. Whencertain conditions have been met, as will be detailed with reference toFIGS. 19 and 20, frequency comparator 1770 will use the output of phasesubtractor 1750 as follows. If phase difference signal 1707 is positive,then predicted phase 1706 is higher than sampled phase 1705, meaningthat the measured frequency is lower than the target frequency. If phasedifference signal 1707 is negative, then the measured frequency ishigher than the target frequency. Frequency comparator 1770 providesthis change in phase relative to the initial phase (frequency differencesignal 1708,, which indicates a direction, and which may further includethe magnitude) to search controller 1780. Thus, a frequency comparator1770 measurement cycle provides a direction of frequency differencesignal 1708, which in turn provides a bit for the oscillator controlsignal. Frequency comparator 1770 and search controller 1780 may repeatthis cycle until all bits included in the oscillator control signal havebeen determined. Once the embodiment has determined all bits included inthe oscillator control signal, the search has been completed. Theembodiment may then: copy the oscillator control code, or one or morenumbers derived from it, into loop filter 1760; switch multiplexer 1790to the other position allowing loop filter 1760 to provide oscillatorcontrol signals to controlled oscillator 1710; and for one more timeload sampled phase 1705 into phase predictor 1740 to ensure that lock-incan continue based on a small phase error. In embodiments, an oscillatorcontrol code may be included in an oscillator control signal, or anoscillator control signal may be derived from the oscillator controlcode, for example using a digital-to-analog converter. In someembodiments, search controller 1780 comprises a filter to reduce noise.In further embodiments, search controller 1780 may set parameters ofloop filter 1760.

FIG. 18 illustrates an alternative PLL 1800 with a frequency comparator1870 according to an embodiment of the invention. PLL 1800 furtherincludes controlled oscillator 1810, phase accumulator 1820, register1830, which may be included in phase accumulator 1820, phase predictor1840, phase subtractor 1850, loop filter 1860, register 1875, searchcontroller 1880, and multiplexer 1890. PLL 1800 provides an output clocksignal 1803, which must be based on a reference clock signal 1801 and anFCW. In this embodiment, frequency comparator 1870 does not load thevalue of measured phase 1805 into phase predictor 1840 as an initialphase value, but rather it loads the value of phase difference 1807 intoregister 1875 as an initial phase value and subtracts the register 1875value from phase difference 1807 to obtain a signal that exposes thechange in phase relative to the initial phase. If the signal ispositive, the predicted phase 1806 is larger than the measured phase1805, and vice versa if it is negative. The frequency comparator 1870can work in a similar fashion as described for the embodiment in FIG.17. It can achieve fast lock-in by enabling search controller 1880 toperform a search, such as a binary search, or it can allow PLL 1800 tofunction as a frequency locked loop during the lock-in period. Theembodiment of FIG. 18 has one big advantage over the embodiment of FIG.17 if PLL 1800 incorporates glitch correction in phase subtractor 1850.In such an embodiment, phase difference 1807 is glitch-corrected, andtherefore lock-in can occur with more accurate values. Conversely,sampled phase 1705 is not glitch-corrected, which means that theembodiment in FIG. 17 may lose some lock-in time due to inaccuracies. Insome embodiments, search controller 1880 comprises a filter to reducenoise. In further embodiments, search controller 1880 may set parametersof loop filter 1860.

FIG. 19 illustrates a first method 1900 to observe a change in phaserelative to an initial phase prior to PLL lock according to anembodiment of the invention. First method 1900 assumes a PLL asillustrated in FIG. 17 or FIG. 18. First method 1900 comprises thefollowing steps.

Step 1910—capturing an initial phase to allow observing a change inphase relative to the initial phase. An embodiment may achieve this byloading sampled phase 1705 as an initial phase into phase predictor1740. As a result, subsequent values of predicted phase 1706 provide thechange in phase relative to the initial phase. Another embodiment mayachieve this by loading a value of the phase difference 1807 as aninitial phase into register 1875, so that the change in phase relativeto the initial phase can be observed by subtracting the value in theregister 1875 from the phase difference 1807.

Step 1920—running the PLL for one or more cycles of reference clocksignal 1701 or reference clock signal 1801. Embodiments may use asufficiently large number of cycles to reduce the relative impact of anynoise components.

Step 1930—upon completing the one or more cycles, observing the changein phase relative to the initial phase to output a signal related to adifference between target frequency and oscillator frequency. The signalmay include the sign and optionally the magnitude of phase differencesignal 1707 or phase difference 1807. If the sign is positive, then thecontrolled oscillator 1710 or controlled oscillator 1810 frequency istoo low, and if it is negative, then the controlled oscillator 1710 orcontrolled oscillator 1810 frequency is too high. In some embodiments,the magnitude of the frequency difference equals the phase differencesignal 1707 magnitude times the reference clock signal 1701 frequency,divided by the number of the one or more cycles of reference clocksignal 1701.

FIG. 20 illustrates a second method 2000 to observe a change in phaserelative to the initial phase prior to PLL lock according to anembodiment of the invention. Second method 2000 assumes a PLL asillustrated in FIG. 17 or FIG. 18. Second method 2000 comprises thefollowing steps.

Step 2010—capturing an initial phase to allow observing a change inphase relative to the initial phase. An embodiment may achieve this byloading sampled phase 1705 as an initial phase into phase predictor1740. As a result, subsequent values of predicted phase 1706 provide thechange in phase relative to the initial phase. Another embodiment mayachieve this by loading a value of the phase difference 1807 as aninitial phase into register 1875, so that the change in phase relativeto the initial phase can be observed by subtracting the value in theregister 1875 from the phase difference 1807.

Step 2020—running the PLL and counting a number Y of reference clocksignal 1701 cycles until the phase difference signal 1707 magnitudeexceeds a threshold (or of reference clock signal 1801 cycles until thefrequency comparator 1870 input signal magnitude exceeds a threshold),or until a timeout is reached. An embodiment may determine the timeoutby comparing Y with a timeout value Y_(MAX), or by any other methodknown in the art to measure the lapse of time. Embodiments use asufficiently large threshold to reduce the relative impact of any noisecomponents.

Step 2030—upon exceeding the threshold, determining sign and optionallythe magnitude of phase difference signal 1707 or of the frequencycomparator 1870 input signal. If the signal is positive, then thecontrolled oscillator 1710 or 1810 frequency is too low, and if it isnegative, then the controlled oscillator 1710 or 1810 frequency is toohigh. The magnitude of the frequency difference equals the phasedifference signal 1707 or frequency comparator 1870 input signalmagnitude times the reference clock signal 1701 frequency, divided by Y.

Step 2040 (optional)—upon reaching timeout, declaring the frequencydifference to be zero.

PLL with Phase Range Extension

In some applications, the momentary phase of a PLL's reference clock maydeviate very substantially from its average value. For example, theITU-T Recommendation G.813 specifies that a reference clock pulse maycome from 2.5 μs early to 2.5 μs late. A PLL's DCO clock frequency maybe, for example, 15 GHz, having a cycle time of 66.7 ps. To measure thereference clock's phase error (i.e., phase difference) in full cycles ofthe DCO clock would mean measuring 37,500 cycles, which requires atleast a 17-bit counter. As was discussed previously in this patentdocument, a counter with many bits may consume much power. Embodimentsof the invention avoid this by using a phase accumulator, phasepredictor and phase subtractor with a limited unextended range andkeeping track of the number of times that an excursion occurs of theunextended range. Some embodiments focus on excursions where the phasedifference exceeds one end of the limited unextended range and is mappedthrough the other end. For example, an upward excursion may occur whenthe phase difference exceeds the top of the range and a downwardexcursion occurs when the phase difference exceeds the bottom of therange. However, other embodiments focus on upward and downwardexcursions of another range threshold point, as will be explained withreference to FIG. 24. While keeping track of excursions in a circularnumbering system, downward excursions compensate for upward excursions,and vice versa. A circular numbering system is a numbering system inwhich the code for the lowest represented number can be obtained byincrementing the code for the highest represented number by 1, and viceversa.

Although systems supporting a standard such as G.813 may have huge phaseerrors, these phase errors build slowly over time as the frequencyvariation of the DCO clock is kept very small. As a result, the valuesfor successive phase errors will be very close to each other unless anexcursion occurs. This happens very infrequently, and it can be detectedusing simple circuitry with a small bit width. Therefore, embodimentsmay provide an ultra-wide range extension using very little power.

One embodiment remedies the situation with the power-saving phaseaccumulator 900 discussed with reference to FIG. 9. However, FIGS. 21-28show embodiments that may save even more power.

FIG. 21 illustrates pseudo code 2100 for a PLL with range extensionaccording to an embodiment of the invention. Pseudo code 2100 includeslines numbered 2101-2114. Pseudo code 2100 centers around a phasecomparator (phase subtractor) whose output is a phase difference(phase_difference). The phase difference is expressed as a 2'scomplement code with a minimum value −K/2 and a maximum value K/2−1,wherein K expresses an unextended phase range, and pseudo code 2100focuses on excursions at the top and bottom of the phase differencerange, i.e. it has range threshold points at −K/2 and K/2−1. Theembodiment uses a parameter (range_ext_steps) to track range excursions,the parameter denoting the number of times the phase difference hasexceeded its lower range threshold point −K/2 minus the number of timesthe phase difference has exceeded its upper range threshold point K/2−1.In line 2101, the embodiment initializes range_ext_steps to zero. Theembodiment may then enter a loop including lines 2102 to 2114.

The embodiment remembers a prior value (last_phase_difference) in line2102. It updates phase_difference in line 2103 in the manner explainedin previous embodiments by taking a difference between a predicted phase(phase_prediction) from a phase predictor and a measured phase(phase_measurement) from a phase accumulator. However, the embodimentadds a value calculated from the multiplication of unextended phaserange K and range_ext_steps. In line 2104, the embodiment calculates adifference (phase_diff_diff) between the present value of the phasedifference and the prior value.

In line 2106, the embodiment compares phase_diff_diff with the upperthreshold point K/2−1. If phase_diff_diff is a larger number, then theembodiment decrements range_ext_steps by 1 (line 2107), and decrementsphase_difference by K (line 2108).

In line 2110, the embodiment compares phase_diff_diff with the lowerthreshold point −K/2. If phase_diff_diff is a smaller number, then theembodiment increments range_ext_steps by 1 (line 2111) and incrementsphase_difference by K (line 2112).

An example manner in which an embodiment determines whether the phasedifference exceeds its range is as follows. When phase errors buildslowly over time, the values for phase_difference andlast_phase_difference will generally be very close to each other.Therefore, phase_diff_diff will usually be close to zero, or generally,a valid range for phase_diff_diff will be dependent on system designparameters, and can therefore be known to be, for example, from −K/4 toK/4 in one application, or from −K/1000 to K/1000 in anotherapplication. However, when a range excursion occurs, phase_differenceand last_phase_difference end up on opposite sides of the phase range,in this example −K/2 to K/2−1, and phase_diff_diff will significantlyexceed its known valid range. The direction of the range excursiondictates the sign of phase_diff_diff. The range excursion causes theamplitude of phase_diff_diff to be outside of its valid range. Someembodiments may test phase_diff_diff strictly for exceeding its knownvalid range. Other embodiments, including the example embodiment 2100,add some margin to this, for example to reduce noise, and test at largerlimits, such as −K/2 and K/2−1.

Although some embodiments may calculate phase_diff_diff by subtractingall last_phase_difference bits from all phase_difference bits, most ofthe bits may have no impact on the result. In an embodiment, only a fewof the most significant bits, for example the two most significant bitsof the unextended phase range, may impact the result. Therefore, someother embodiments may determine only those few most significant bits ofphase_diff_diff, and subtract only the few most significant bits oflast_phase_difference from the few most significant bits ofphase_difference. Further embodiments may only store the few mostsignificant bits of last_phase_difference, as the remaining bits have noimpact on phase_diff_diff.

FIG. 22 illustrates alternative pseudo code 2200 for a PLL with rangeextension according to an embodiment of the invention. Pseudo code 2200includes lines numbered 2201-2214. Pseudo code 2200 is different thanpseudo code 2100 in that it uses values of the current and previousunextended phase difference to determine if an excursion occurs.

FIG. 23 illustrates an example PLL 2300 with range extension for anarbitrary range value K according to an embodiment of the invention.FIG. 23 implements the method of FIG. 22. As in previous embodiments, Kis the larger of a maximum phase to be measured in a phase accumulatorand a maximum phase to be predicted in a phase predictor. In thisexample, an unextended phase range may span from −K/2 to K/2−1, as in atwo's complement code, or more generally, in a circular numbering systemthe range may span from the lower threshold point to the higherthreshold point. PLL 2300 includes phase accumulator 2320, measuredphase register 2330 (which may be included in phase accumulator 2320),phase predictor 2340, phase subtractor 2350, last phase differenceregister 2352, difference difference subtractor 2354, bottom excursioncomparator 2356, top excursion comparator 2357, range extension stepsregister 2358 which may be included in a counter, multiplier 2359, andphase extension adder 2360. Phase accumulator 2320, measured phaseregister 2330, and phase predictor 2340 function as in any otherembodiment. Phase subtractor 2350 subtracts a measured phase register2330 output value from a phase predictor 2340 output value to calculatean unextended phase difference 2351. Last phase difference register 2352holds and outputs an unextended phase difference previous value 2353.Difference difference subtractor 2354 calculates a phase_diff_diff value2355 by subtracting unextended phase difference previous value 2353 fromunextended phase difference 2351. Bottom excursion comparator 2356determines if an excursion occurs at the lower threshold point bycomparing phase_diff_diff value 2355 with −K/2. If phase_diff_diff value2355 is less than the lower threshold point −K/2, then bottom excursioncomparator 2356 triggers an increment input of range extension stepsregister 2358. Top excursion comparator 2357 determines if an excursionoccurs at the higher threshold point by comparing phase_diff_diff value2355 with K/2. If phase_diff_diff value 2355 is larger than or equal tohigher threshold point K/2, then top excursion comparator 2357 triggersa decrement input of range extension steps register 2358. Whentriggered, range extension steps register 2358 increments or decrementsits output value (the range_ext_steps parameter) accordingly. Multiplier2359 takes this output value, and multiplies it by K. Phase extensionadder 2360 adds the multiplier 2359 output value to the unextended phasedifference 2351 to obtain an extended phase difference 2361 value thatthe embodiment forwards to a loop filter. The range of extended phasedifference 2361 depends on the bit width of range extension stepsregister 2358. If range extension steps register 2358 has a range of L,then the range of extended phase difference 2361 is K* L, for examplefrom −L*K/2 to L*K/2−1.

Some embodiments may use only a few significant bits, for example thetwo most significant bits, of the unextended phase difference 2351 tostore in last phase difference register 2352 and subtract fromunextended phase difference previous value 2353 in the differencedifference subtractor 2354. In those embodiments, the bit width ofphase_diff_diff value 2355 will be reduced and both bottom excursioncomparator 2356 and top excursion comparator 2357 will operate withfewer bits to determine if a range excursion occurs. For example,embodiments with K=48 or K=64 would associate a bit width of 6 bits withK. If only the two most significant bits impact a range excursion, thenphase subtractor 2350 would be 6 bits wide, whereas last phasedifference register 2352, difference difference subtractor 2354, bottomexcursion comparator 2356, and top excursion comparator 2357 would be 2bits wide. Range extension steps register 2358 may have any number ofbits to allow for an ultra-wide extended phase range.

FIG. 24 illustrates how a PLL can alternatively provide range extensionaccording to an embodiment of the invention. The method works when K isa whole power of two, such as 2, 4, 8, etc. In the example in FIG. 24,the PLL's phase accumulator and phase predictor have a small(unextended) range with K=4, in this case from −2 to +1. The numberingsystem is circular, and in this case two's complement. As will be shown,a range threshold point in between the top and the bottom of the rangecan be used fruitfully. FIG. 24 shows table 2400, whose first columnshows an extended range in decimal numbers. The extended range is alsokept small, from −8 to +7. The bits produced by the phase subtractor areshown in the third column. The second column shows the two bitsconcatenated as most significant bits and stored in a range extensionregister, which may be included in an up/down counter. As can be seenfrom table 2400, the bits in the second column concatenated with thebits in the third column provide the two's complement code for theextended range shown as a decimal number in the first column.

Although the phase subtractor wraps around from a two's complement value01 (=+1) to 10 (=−2) or vice versa, the extended bits remain the samefor those values. Such wrap arounds occur in the extended range at 5 and6; 1 and 2; −2 and −3; and −6 and −7. Nothing needs to be done toachieve range extension at these points. However, the extended rangebits change at 4 and 3; 0 and −1; and −4 and −5, threshold points in themiddle of the phase difference range. For example, an upward excursionoccurs when the extended range bits increment (second column, from 00 to01) when the extended range phase difference goes from 3 to 4. They alsoincrement (and upward excursions occur) (second column from 11 to 00)when the extended range phase difference goes from −1 to 0, and (secondcolumn from 10 to 11) when the extended range phase difference goes from−5 to −4. In all these cases, the phase difference in the third columngoes from 11 (prior value equals −1) to 00 (current value equals 0).From this, it can be concluded that an upward excursion occurs and thebits in the range extension register must increment when the prior valueequals −1 and the current value equals 0. Conversely, a downwardexcursion occurs and the bits in the range extension register mustdecrement when the prior value equals 0 and the current value equals −1.Therefore, FIG. 24 shows that bits in the range extension register canbe concatenated with unextended phase difference bits to obtain anextended phase difference using a single range threshold point whosevalue lies between 0 and −1. It must be noted that even when the K valueis higher (e.g. 8, 16, 32, etc.), the excursion can be detected bylooking at only the two most significant bits of the phase difference.The table of FIG. 24 remains valid for any number of additional moresignificant bits of the range extension, or less significant bits of theunextended phase range.

FIG. 25 illustrates pseudo code 2500 for the example in FIG. 24 for aPLL with range extension according to an embodiment of the invention.The method begins in line 2501 by initializing the range extensionregister to 0 (or to another predetermined value). It then enters a loopfrom line 2502 to 2513 that begins with updating a prior phasedifference register in line 2502. The register may hold as few as thetwo most significant bits of the phase difference. Line 2503 determinesan updated phase difference by subtracting a measured phase (provided bythe phase accumulator) from a predicted phase (provided by the phasepredictor). Line 2504, which is optional, determines the at least twomost significant bits of the phase difference. Lines 2506-2508 determineif a downward excursion occurs (prior value equals 0 and the currentvalue equals −1), and lines 2509-2511 determine if an upward excursionoccurs (prior value equals −1 and the current value equals 0). Line 2512determines the extended phase difference by adding K times the rangeextension register value to the phase difference. As discussed, themultiplication and addition can be achieved at the same time byconcatenating the range extension register bits as most significant bitsto the (unextended) phase difference bits.

FIG. 26 illustrates an example PLL 2600 with range extension for themethod in FIGS. 24 and 25 according to an embodiment of the invention.PLL 2600 includes phase accumulator 2620, measured phase register 2630,phase predictor 2640, phase subtractor 2650, last phase differenceregister 2652, first current value comparator 2654, second current valuecomparator 2655, first prior value comparator 2656, second prior valuecomparator 2657, decrement detector gate 2658, increment detector gate2659, and range extension register 2662 which may be included in anup/down counter. Signals include unextended phase difference 2651, priorunextended phase difference 2653, decrement signal 2660, incrementsignal 2661, and extended phase range bits 2663. As in other PLLsdescribed in this document phase accumulator 2620 and phase predictor2640 provide the signals for phase subtractor 2650, which calculates theunextended phase difference 2651 upon receiving a reference clocksignal. Last phase difference register 2652 holds a prior version of atleast the two most significant bits of unextended phase difference 2651.First current value comparator 2654, first prior value comparator 2656,and decrement detector gate 2658 determine if a decrement of the valuein range extension register 2662 needs to occur by determining that theprevious phase difference was higher than the range threshold point andthe current phase difference is lower than the range threshold point.Second current value comparator 2655, second prior value comparator2657, and increment detector gate 2659 determine if an increment of thevalue in range extension register 2662 needs to occur by determiningthat the previous phase difference was lower than the range thresholdpoint and the current phase difference is higher than the rangethreshold point. Decrement detector gate 2658 decrements and incrementdetector gate 2659 increments range extension register 2662. Theembodiment concatenates the bits in range extension register 2662 to thebits of unextended phase difference 2651, and present the resultingextended phase range to the PLL's loop filter.

FIG. 27 illustrates generalized circuitry 2700 for phase differencerange extension according to an embodiment of the invention. FIG. 27 isderived from FIG. 23 and FIG. 26 on the basis that the function ofcomparators 2356-2357 or 2654-2657 and AND gates 2658-2659 can beachieved with different (but equivalent) networks of combinatoriallogic, and that in fact a silicon compiler program will translate thecircuits as presented here to one that is optimized for thesemiconductor and transistor technology available to an IC designer.Circuitry 2700 includes phase accumulator 2720, phase predictor 2740,phase subtractor 2750, first register 2752, second register 2762 whichmay be included in an up/down counter, and combinatorial logic network2757. Some embodiments further include scaler 2764 and adder 2766. Uponreceiving a reference clock signal, phase subtractor 2750 determinesunextended phase difference 2751 by subtracting a measured phasedelivered by phase accumulator 2720 from a predicted phase delivered byphase predictor 2740. Combinatorial logic network 2757 uses two or moremost significant bits from unextended phase difference 2751 and two ormore most significant bits from the previous value of unextended phasedifference 2751 stored in first register 2752 to determine if a phasedifference excursion occurs, which may be an upward excursion or adownward excursion, based on one or two range threshold points. When anupward excursion occurs, combinatorial logic network 2757 asserts anincrement signal (inc), and when a downward excursion occurscombinatorial logic network 2757 asserts a decrement signal (dec). Theinc signal increments the value 2763 stored in second register 2762,whereas the dec signal decrements it. The embodiment multiplies value2763 by K in scaler 2764 and adds the result 2765 to unextended phasedifference 2751 in adder 2766 to calculate an extended phase difference2767 that the PLL presents to a loop filter. Some embodiments (thosewith two range threshold points) achieve this multiplication andaddition by using dedicated logic, i.e., scaler 2764 and adder 2766.Embodiments with a single range threshold point may achieve thismultiplication and addition by concatenating the bits of value 2763 tothe bits of unextended phase difference 2751, where the bits of value2763 become the most significant bits and the bits of unextended phasedifference 2751 become the less significant bits of extended phasedifference 2767.

FIG. 28 illustrates a generalized method 2800 to achieve phasedifference range extension according to an embodiment of the invention.Method 2800 is suitable for use with the circuitry 2700 in FIG. 27, orthe more specific versions in FIGS. 23 and 26. Method 2800 includes thefollowing steps.

Step 2810—initializing a first register and a second register to zero.The first register may be two or more bits wide and will be used forstoring two or more bits of the unextended phase difference. The secondregister has a bit width that determines by which factor the unextendedphase difference range is extended. For example, if the unextended phasedifference range equals K, and the second register has a width of 12bits (i.e., its range equals 2¹²=4096), then the extended phasedifference range will be 4096 times K. Some embodiments may initializethe first register or the second register to a different value thanzero.

Step 2820—upon receiving a reference clock signal, determining a currentunextended phase difference by subtracting a measured phase from apredicted phase, and retrieving two or more most significant bits from aprevious unextended phase difference stored in the first register.

Step 2830—in a combinatorial logic network, using two or more mostsignificant bits from the current unextended phase difference and theretrieved two or more most significant bits from the previous unextendedphase difference to determine if an excursion occurs. There are threepossible outcomes: (1) no range excursion occurs; (2) an upwardexcursion occurs; or (3) a downward excursion occurs. Excursions may bebased upon a single range threshold point situated in between the bottomand top of the phase difference range as illustrated in FIG. 24, or tworange threshold points, one situated below the bottom of the range andone situated above the top of the range as illustrated in FIGS. 21 to23.

Step 2840—upon determining that an upward excursion occurs, incrementinga value stored in the second register.

Step 2850—upon determining that a downward excursion occurs,decrementing the value stored in the second register.

Step 2860—storing the two or more most significant bits of the currentunextended phase difference in the first register and calculating anextended phase difference. Some embodiments determine the extended phasedifference by concatenating the bits stored in the second register tothe bits of the unextended phase difference. Other embodiments determinethe extended phase difference by multiplying the value stored in thesecond register with the value of the unextended phase difference rangeand adding it to the current unextended phase difference.

General

It will be understood that the invention disclosed and defined in thisspecification extends to all alternative combinations of two or more ofthe individual features mentioned or evident from the text or drawings.All of these different combinations constitute various alternativeaspects of the invention.

Although the description has been described with respect to particularembodiments thereof, these particular embodiments are merelyillustrative, and not restrictive. For instance, many of the operationscan be implemented on a printed card board PCB using off-the-shelfdevices, in a System-on-Chip (SoC), application-specific integratedcircuit (ASIC), programmable processor, or in a programmable logicdevice such as a field-programmable gate array (FPGA), obviating a needfor at least part of the dedicated hardware. All such variations andmodifications are to be considered within the ambit of the presentinvention the nature of which is to be determined from the foregoingdescription.

Any suitable technology for manufacturing electronic devices can be usedto implement the circuits of particular embodiments, including bipolar,JFET, MOS, NMOS, PMOS, CMOS, BiCMOS, HBT, MESFET, FinFET, etc. Differentsemiconductor materials can be employed, such as silicon, germanium,SiGe, GaAs, InP, graphene, etc. Circuits may have single-ended ordifferential inputs, and single-ended or differential outputs. Terminalsto circuits may function as inputs, outputs, both, or be in ahigh-impedance state, or they may function to receive supply power, aground reference, a reference voltage, a reference current, or other.Although the physical processing of signals may be presented in aspecific order, this order may be changed in different particularembodiments. In some particular embodiments, multiple elements, devices,or circuits shown as sequential in this specification can be operatingin parallel.

Particular embodiments or parts of an embodiment may be implemented in atangible, non-transitory computer-readable storage medium for use by orin connection with an instruction execution system, apparatus, system,or device. Particular embodiments can be implemented in the form ofcontrol logic in software, firmware, hardware or a combination of those.The control logic, when executed by one or more processors, may beoperable to perform that which is described in particular embodiments.For example, a tangible medium such as a hardware storage device can beused to store the control logic, which can include executableinstructions.

It will also be appreciated that one or more of the elements depicted inthe drawings/figures can also be implemented in a more separated orintegrated manner, or even removed or rendered as inoperable in certaincases, as is useful in accordance with a particular application. It isalso within the spirit and scope to implement a program or code that canbe stored in a machine-readable medium to permit a computer to performany of the methods described above.

As used in the description herein and throughout the claims that follow,“a”, “an”, and “the” includes plural references unless the contextclearly dictates otherwise. Also, as used in the description herein andthroughout the claims that follow, the meaning of “in” includes “in” and“on” unless the context clearly dictates otherwise.

Thus, while particular embodiments have been described herein, latitudesof modification, various changes, and substitutions are intended in theforegoing disclosures, and it will be appreciated that in some instancessome features of particular embodiments will be employed without acorresponding use of other features without departing from the scope andspirit as set forth. Therefore, many modifications may be made to adapta particular situation or material to the essential scope and spirit.

1. A phase-locked loop (PLL), comprising: a phase predictor; a phasesubtractor; and a frequency comparator configured to output a signalrelated to a difference between a target frequency and an oscillatorfrequency by capturing an initial phase and observing a change in phaserelative to the initial phase; wherein the frequency comparator isactive during a lock-in period.
 2. The PLL of claim 1, wherein thecapturing of the initial phase is performed by loading a value of asampled phase that represents the initial phase in the phase predictor,such that subsequent predictions provided by the phase predictor includethe change in phase relative to the initial phase.
 3. The PLL of claim1, wherein the capturing of the initial phase is performed by loading avalue of a phase difference into a register and wherein the change inphase relative to the initial phase is calculated by subtracting thevalue in the register from subsequent values of the phase difference. 4.The PLL of claim 1, wherein the frequency comparator observes only asign of the change in phase relative to the initial phase to determineif the oscillator frequency is higher than the target frequency.
 5. ThePLL of claim 1, wherein the frequency comparator waits for one or morecycles of a reference clock signal before observing the change in phaserelative to the initial phase.
 6. The PLL of claim 1, wherein thefrequency comparator only outputs the signal if a magnitude of thechange in phase relative to the initial phase exceeds a threshold. 7.The PLL of claim 6, wherein the frequency comparator outputs a signalthat the difference equals zero if a timeout has occurred.
 8. The PLL ofclaim 1, further comprising a search controller configured for findingan oscillator control signal that results in an oscillator frequencywithin a search margin of the target frequency using successive outputsignals of the frequency comparator.
 9. The PLL of claim 8, wherein thesearch controller finds the oscillator control signal by performing oneof a binary search, a linear search, a jump search, an interpolationsearch, an exponential search, a Fibonacci search, and a Newton-Raphsonsearch.
 10. The PLL of claim 8, wherein the search controller comprisesa filter.
 11. The PLL of claim 8, wherein the search controller setsparameters of a loop filter.